In insulated gate bipolar transistors (IGBTs), insulated gate field effect transistor (IGFET) cells control a base current of a bipolar junction transistor (BJT). Minority carriers injected from a collector region of the BJT into a drift region during forward conduction considerably reduce the on-state resistance of the low doped drift region. In an superjunction IGBT (SJ-IGBT) or superjunction bipolar transistor (SJ-BT), a superjunction structure including alternatingly arranged and more strongly doped p-type and n-type semiconductor areas replaces at least a portion of the drift region. Since the electric field builds up not only in a vertical direction but also in a horizontal direction parallel to main surfaces of the SJ-IGBT, the breakdown voltage of SJ-IGBTs to a lower degree dependents on the dopant concentration in the drift region assumed that the p-type semiconductor areas and n-type semiconductor areas compensate each other sufficiently well and are completely depleted before breakdown takes place. Unlike in superjunction IGFETs both types of semiconductor areas contribute to a current flow and affect the switching behavior.
It is desirable to improve the switching behavior of superjunction bipolar transistors such as SJ-IGBTs.